1. Field of the Invention
The present invention relates to a process for cracking deep cut vacuum gas oils, resids or other reduced crudes for increased gasoline octane utilizing a catalytic cracking catalyst, which includes a ZSM-5 component that said ZSM-5 component exhibits a surprising tolerance to poisoning by sodium and vanadium.
2. Background of the Related Art
Current worldwide refinery trends indicate a continuing need to process heavier feedstocks. As a result, many refineries will be processing FCC feeds containing resids or deeper cut gas oils which have high metals content. The phaseout of lead additives for gasoline in both the U.S. and in Europe will require refiners to turn to other alternatives to increase gasoline octane, including octane catalysts.
Fluid catalytic cracking (FCC) is commercially practiced in a cycling mode in which a hydrocarbon feedstock is contacted with hot, active, solid particular catalyst without added hydrogen at rather low pressures of up to about 50 PSIG at temperatures sufficient to support the desired cracking. As the hydrocarbon feed is cracked to form more valuable and desirable products, in the presence of a cracking catalyst, carbonaceous residue known as "coke" is deposited on the catalyst. The coke contains carbon as well as metals which are present in the feedstock.
In fluid catalytic cracking (FCC), the catalyst is a fine powder of about 20-200 microns in size. The fine powder is propelled upwardly through a riser reaction zone, suspended and thoroughly mixed in the hydrocarbon feed. The hydrocarbon feed is cracked at high temperatures by the catalyst and separated into various hydrocarbon products. The coked catalyst particles are separated from the cracked hydrocarbon products, and after purging, are transferred into the regenerator where the coke is burned off to regenerate the catalyst. The regenerated catalyst then flows downward from the regenerator to the base of the riser.
Deposition of coke on the catalyst particles is generally considered undesirable for two reasons: first, it inevitably results in a decline in catalytic activity to a point where the catalyst is considered to have become "spent"; and second, coke generally forms on the catalyst at the expense of the more desired liquid products. To regenerate the catalytic activity, the hydrocarbon residues of the coke must be burnt off of the "spent" catalyst at elevated temperatures in the regenerator. Unlike the hydrocarbon residues, the metallic residues such as vanadium, sodium, nickel and the like, are not removed by high temperature regeneration. Rather, the catalytic cracking catalyst must be somehow protected from the deleterious effects of metallic residues.
There are substantial economic incentives for refineries to process reduced crudes due to factors such as the price and availability of heavy fuel oil ("HFO"), coal, the demand for asphalt and a particular refiner's ability to process the 650+ through 1150+ fractions.
Reduced crudes, however, are generally high in metals which poison the cracking catalyst and therefore require a high replacement rate of the catalyst in order to maintain catalyst selectivity and activity. In recent years, extensive research efforts have been directed towards developing improved catalysts and processes for reducing the deleterious effects caused by metals when cracking resids or deep cut vacuum gas oils.
The use of ZSM-5 type zeolites, such as ZSM-5, ZSM-11 and the like are known to result in improved gasoline octane and overall yields when used in conjunction with conventional cracking catalysts in cracking gas oils. The use of ZSM-5 in cracking is disclosed in U.S. Pat. Nos. 3,702,886; 3,758,403; 3,894,931; 3,894,933; 3,894,934; 4,309,279; 4,309,280; and 4,416,765. As understood, none of these patents specifically teach the use of ZSM-5 in resid cracking. While more recent patent, U.S. Pat. No. 4,552,648 includes examples which use ZSM-5 with hydrotreated Arab Light Resid, the chargestock contained less than 1 ppm Ni and V. In addition, this patent does not address the unexpected and exceptional metals tolerance of ZSM-5 catalysts to poisoning by these metals.
The deleterious effects of metals have been mentioned extensively in the patent literature, for example U.S. Pat. Nos. 4,376,696; 4,513,093 and 4,515,900, and are well known to those skilled in the art. Vanadium substantially deactivates cracking catalysts by irreversibly destroying the active zeolite, while other metals such as nickel promote dehydrogenation reactions which result in undesirable increases in coke and hydrogen yields.
It has long been known to those in the petroleum refining art that the combination of sodium and vanadium in a crude feedstock, results in a synergistically destructive effect on the Y-containing cracking catalysts. For example, in the presence of about 0.5 wt % Na, a vanadium level of 5,000 ppm on the catalyst will have a substantially greater destructive effect on the Y-containing zeolite under hydrothermal conditions than when there is no sodium present. A priori, a similar synergistically destructive effect might also be expected by the combination of sodium and vanadium on ZSM-5 type catalysts.
It has been previously discovered that ZSM-5 type zeolites used in conjunction with conventional cracking catalysts, exhibit a high tolerance to metal poisoning by vanadium and nickel present in resids or other reduced crudes. The results of this work were disclosed at the fourth CCIC Technical Meeting, Tokyo, Japan on June 9, 1986 in a paper entitled ZSM-5 IN FCC, POTENTIAL IMPACT ON REFINERY OPERATIONS, by F. G. Dwyer, F. Gorra, J. Herbst, Mobil Research and Development Corporation, Paulsboro, N.J. 08066, U.S.A. Briefly, the paper disclosed the excellent tolerance of ZSM-5 to vanadium and nickel poisoning in metal-containing crudes and its resulting advantages and applications in processing resids or other reduced crudes. This publication, however, does not disclose the unexpected resistance of ZSM-5 to the synergistically destructive combination of sodium and vanadium.
Octane enhancement in FCC and the excellent metal tolerance by ZSM-5 was again the topic of a presentation at the NPRA ANNUAL MEETING held on Mar. 29-31, 1987, in San Antonio, Tex.. This presentation was published by F. G. Dwyer and P. H. Schipper of Mobil Research and Development Corporation, Paulsboro, New Jersey and F. Gorra of EniChem Anic SA, Milan, Italy, in a document entitled: Octane Enhancement in FCC Via ZSM-5, which is incorporated by reference herein. The document discusses the results of over 25 FCC and TCC operations in the United States and abroad, evaluating octane enhancement by ZSM-5, and contains an excellent background discussion of the use of ZMS-5 in FCC and TCC units. This document, at page 6, is the first to present results of laboratory tests conducted by the inventors herein, with ZSM-5 as an additive catalyst, poisoned by the combination of sodium and vanadium. The results of these tests, summarized in Table 7 of that document, show no change in the octane enhancing performance of the ZSM-5 catalyst.
Accordingly, the present invention sets forth a solution to a long felt need in the petroleum refining industry by providing a method for upgrading resids containing high concentrations of metals such as nickel, sodium and vanadium to increase gasoline octane by utilizing a catalytic cracking catalyst, which includes a ZSM-5 component. The ZSM-5 component of the catalyst not only exhibits surprising tolerance to metal poisoning due to nickel and vanadium, but also quite unexpectedly, exhibits tolerance to the normally synergistically destructive combination of sodium and vanadium on Y-type zeolites.
SUMMARY OF THE INVENTION
Accordingly, the present invention provides a process for catalytically cracking vacuum gas oils, resids, or other reduced crudes containing metal contaminants to increase gasoline octane. Hydrocarbon feedstocks which include vanadium and sodium contaminants are introduced into the reaction zone of a catalytic cracking unit. A catalytic cracking catalyst which includes a ZSM-5 type catalyst component is fluidized in the reaction zone in contact with the metals-containing hydrocarbon feedstock. The hydrocarbon feedstock is cracked at high temperatures by the cracking catalyst, resulting in increased gasoline octane of the hydrocarbon product, and surprising tolerance of the ZSM-5 type catalyst component to poisoning from the normally expected synergistically destructive combination of sodium and vanadium on Y-type zeolites.
In addition to the ZSM-5 type component, the cracking catalysts utilized in the process of the present invention also typically contain catalytically active cracking components having a pore size greater than about 7 angstroms. Such components include amorphous silica-alumina, and/or crystalline silicaalumina and/or large pore crystalline zeolites.
Representative crystalline zeolite constituents of these cracking catalysts include zeolite X described in U.S. Pat. 2,882,244, zeolite Y described in U.S. Pat. 3,130,007, synthetic mordenite and dealuminized synthetic mordenite, merely to name a few, as well as naturally occurring zeolites, including chabazite, faujasite, mordenite, and the like. Preferred crystalline zeolites include natural faujasite and the synthetic faujasite zeolites X and Y, with particular preference being accorded to zeolite Y. For the purposes of this invention, zeolite Y includes zeolite Y in its as-synthesized form, as well as its variant forms including framework dealuminated zeolite Y, e.g., ultrastable Y (USY) described in U.S. Pat. No. 3,293,192 and LZ-210 described in U.S. Pat. No. 4,503,023.
The hydrocarbon feedstock may also include the combination of nickel and sodium, or nickel and vanadium. Preferably, the reaction zone of a catalytic cracking unit is maintained at a temperature from about 900.degree. F. to about 110.degree. F. and the catalyst regenerator temperature is maintained at about 1200.degree.-1400.degree. F. The hydrocarbon feedstock which is introduced into the reaction zone of the catalytic cracking unit, typically contains from about 0.1 to about 25 lbs. sodium and from about 0.5 to about 250 lbs. vanadium per thousand barrels. The ZSM-5 type component of the catalytic cracking catalyst is replaced at a makeup rate of about 0.2 lbs. to about 500 lbs. ZSM-5 per thousand barrels of feedstock processed. Typically, the ZSM-5 type component is present in a range from about 0.5 to about 5% of the catalytic cracking catalyst inventory.